This invention relates generally to optical communication devices and arrangements, and, in particular, to an improved push-pull arrangement for a thermo-optic switch.
Wavelength Division Multiplexing (WDM) control devices, such as wavelength add-drops (WADs), wavelength selective cross connects (WSCs), and dynamic gain equalization filters (DGEFs), often consist of a demultiplexer and a multiplexer connected by an array of switches. A low-loss, compact, mass-produceable way to make the switches is to use a planar arrangement of thermo-optic Mach-Zehnder (M-Z) interferometer switches in silica waveguides.
The conventional way to make the switch, as described, for example, in M. Okuno, N. Takato, T. Kitoh, and A. Sugita, xe2x80x9cSilica-based thermo-optic switches,xe2x80x9d NTT Review, vol. 7, no. 5, pp. 57-63, 1995, is to place a thermo-optic phase shifter 110 in one arm 101 of the interferometer, as shown in FIG. 1. A thermo-optic phase shifter is simply a heater deposited over the waveguide that causes the refractive index of the waveguide material to change via a temperature change when electrical current is sent through the heater. Usually the two path lengths (i.e., the lengths of arms 101 and 102) between the input coupler 120 and the output coupler 130 are designed to be equal when the thermo-optic phase shifter is undriven, although sometimes there is a half-wavelength bias.
The conventional arrangement has several drawbacks. The power consumption is high, the total power dissipated changes with the number of activated switches leading to temperature control problems, the polarization dependence is significant, and the phase changes when the switch state is changed.
All four of the problems just described can be mitigated by changing the thermo-optic interferometer switch to become push-pull, by placing approximately a quarter-wavelength effective path-length difference (90 degree bias) between the arms of an interferometer switch in the zero-drive state, and then driving one arm to activate the switch to one state (e.g., the bar state) and driving the other arm to go to the other state (e.g., the cross state).